1. Field of the Invention
The present invention relates generally to a rapidly switchable line selector for pulsed lasers, and more particularly to a system that allows forcing successive pulses of a pulsed laser to be on different and distinct spectral lines.
2. Description of the Prior Art
The only prior art known to the inventor that has actually been demonstrated to work are systems using a rapid mechanical motion to change a diffraction grating's alignment. In one prior art system, the rapid mechanical motion of the diffraction grating is done by a drive that changes the angle of the diffraction grating, as with a pointing mirror. In the other known prior art system, a set of diffraction gratings is mounted on the faces of a spinning hexagonal prism. As each of the diffraction gratings comes into the correct alignment angle, the laser is fired on a different spectral line.
The spinning prism arrangement of the prior art has two major disadvantages, which will be discussed in terms of an infrared CO.sub.2 laser. The primary disadvantage is a limitation of the maximum pulse repetition rate. The diffraction grating is changing alignment during the pulse, and if it turns too rapidly, the laser will go out of alignment during the pulse, clipping the pulse, and reducing the optical energy contained in it. The alignment time is not just the time during which there is significant power in the pulse, but must also include the time for the pulse to build up out of the noise. The times involved are about 0.1 to 0.3 .mu.sec for pulse build-up from the noise, 0.1 to 0.2 .mu.sec for the main pulse, and another .mu.sec if the energy in the tail is also to be used, the total time being approximatey 0.2 to 2 .mu.sec.
The alignment tolerance of the laser is about 50 .mu.rad, so if the diffraction grating rotates faster than 50 .mu.rad/.mu.sec, the pulse will be clipped. Significant energy can be extracted with alignment times as small as 0.1 .mu.sec, which allows rotation rates up to 500 .mu.rad/.mu.sec, but with somewhat reduced energy. These numbers convert to 50 pulses per second with full energy at a prism rotation of 8.3 Hz (500 RPM) and 500 pulses per second with reduced energy at a prism rotation of 83 Hz (5000 RPM) for a hexagonal rotating prism. The achievable pulse repetition rate scales with the number of faces on the prism, but so does the physical size of the prism, because the size of a face is determined by the laser beam size, which is approximately 1 centimeter.
A second major disadvantage of this prior art system is the requirement for precise timing of the firing of the laser relative to the angular position of the spinning prism. The alignment tolerance is, as stated above, 50 .mu.rad, and this is achieved with a precision optical encoder wheel on the same shaft as the prism.
The prior art spinning prism system is not without it's advantages. It has the ability for random access to any spectral line by choosing the firing time relative to the angular position of the prism. But this advantage is offset by the previously described disadvantages.
The present invention solves substantially all of the problems of the prior art systems and avoids the disadvantages of the prior art while providing many advantages thereover.